The present invention relates to a hot-rolled titanium steel plate having a tensile strength of 70 kg/mm.sup.2 or more, as well as improved formability and toughness at low temperatures.
The demand for steels having high strength and formability that can be used as structural materials in buildings and industrial machines and facilities has increased these days. To meet this demand, various types of steel have been developed and come to be used commercially. Among them are niobium steels, vanadium steels and titanium steels. Especially, titanium steel, i.e., Ti-containing steel is attractive because of its low manufacturing cost and high tensile strength. However, the toughness, particularly low temperature toughness of titanium steel is markedly degraded in comparison with that of niobium steels and vanadium steels.
On the other hand, because of the necessity of developing new energy resources, the exploitation of gas and oils, for example, is being extensively carried out even under severe environmental conditions. Therefore, there is a demand for structural materials which can be used under such severe conditions. For example, in the case of a high tensile steel plate 4.5 mm or larger in thickness being used in a cold environment, brittle fracture at the portion where plastic deformation has been applied sometimes results. From this viewpoint, too, it is highly desirable to provide a high tensile steel plate which exhibits improved low temperature toughness, making the steel plate feasible to use under severe conditions in a cold environment in addition to improving properties including high tensile strength and formability.
Hot-rolled, high tensile titanium steel plates are characterized by using the precipitation hardening of TiC and forming TiS (C-type inclusion, i.e. globular inclusion) instead of MnS (which is an A-type inclusion, i.e. elongated inclusion) so as to improve the cold formability of the plates. There are disclosed some papers which treat hardening of steel plates caused by the addition of titanium thereto: "Alloying possibilities for increasing strength and toughness of weldable structural steels" by L. Meyer et al and "The role of strong carbide and sulfide forming elements in the manufacture of formable high strength low alloy steels" by M. Korchynsky et al, SYMPOSIUM, LOW ALLOY HIGH STRENGTH STEELS, Nuremberg, May 21-23, 1970, pp. 9-15 and pp. 17-27, respectively, for example.
A method depending upon this technique is disclosed, for example, in Japanese Patent Publication (JPP) No. 45614/80 and No. 47256/82 and Japanese Patent Laid-Open Specification (JPLOS) No. 84422/81 and No. 41325/81.
In accordance with the disclosures made in JPP No. 47256/82 and No. 45614/80, in addition to using the precipitaion hardening of TiC, Ti itself is used effectively by decreasing the contents of sulfur, nitrogen and oxygen. In order to ensure good cold formability, a fine ferrite structure is produced, and for the purpose of preventing the formation of a bainitic structure, the hot-rolled steel is coiled at a controlled temperature in the range of 500.degree. to 680.degree. C. JPLOS No. 41325/81 also discloses coiling at a temperature of 550.degree.-650.degree. C. JPLOS No. 84422/81 discloses the production of titanium-containing steel plates having a ferrite+pearlite structure, which are subjected, after hot rolling, to cold rolling followed by annealing to provide a steel plate with a sufficient degree of tensile strength.
The so produced Ti-containing steel plate in the prior art has not only improved strength but also a very high cold formability as demonstrated by the fact that a sample with its edges finished by machining can withstand a "close" contact bending test according to the JIS (Japanese Industry Standards).
In the bending test according to the JIS, all samples have their edges finished by machining. However, in almost all cases of commercial production of structural members, blanks shorn to a predetermined size are immediately subjected to cold working without trimming the shorn edges. Therefore, from an application point of view, the cold formability of commercial steel plates must be evaluated by the bending performance of samples with as-shorn edges. However, recent studies on commercial production of Ti-containing hot-rolled steel plates have revealed that most of them including the plates produced by the methods described in JPP No. 45614/80 and JPP No. 47256/82 perform very poorly in a bending test with blanks having as-shorn edges and that they develop cracks in the edges during bending.
Taking these facts into account, the present inventor made various studies to improve the formability of blanks of titanium steel plates with untrimmed, as-shorn edges, as well as the low temperature toughness which is typically low with titanium steel plates. As a result, the inventor has found that a Ti-containing steel plate having improved formability and low temperature toughness can be produced from a steel having a specific chemical composition by controlling its microscopic structure. The present invention has been accomplished on the basis of this finding.
The observations obtained during the studies leading to the accomplishment of the present invention are as follows:
Titanium steel plates that have been coiled at the ordinary coiling temperature (ca. 600.degree. C.) after hot rolling can withstand a close contact bending test under the JIS, so it must be concluded that they have good formability. However, if blanks with as-shorn edges are subjected to the same test, it is difficult to avoid bending cracks due to the cracks already present in the as-shorn edges as induced by the brittleness of the ferrite grain boundary peculiar to hot-rolled titanium steel plate. The reason for the great possibility of cracks to develop in the ferrite grain boundary of shorn edges is that hot-rolled titanium steel plates produced by the conventional process depends on the precipitation hardening of TiC that primarily forms after the hot rolling for their high strength. Most of the TiC that is finely precipitated after the rolling occurs within the ferrite grains, and less is found along the grain boundary. Furthermore, coiling the hot-rolled steel plate at high temperature causes cementite to be precipitated along the grain boundary. As a result, the strength of the grain boundary is decreased relative to the matrix in the grains and the toughness of the grain boundary itself may be reduced, and upon shearing, a strain will concentrate in the grain boundary to develop micro cracks in the shorn edges. Such micro cracks serve as starting points for the development of cracks in the ferrite grain boundary in the subsequent bending operation.
It is believed that TiC precipitating after rolling is coherent with the ferrite matrix (viz. is accompanied by a great amount of strain) and easily causes steel embrittlement. Therefore, it is assumed that suppressing the coherent precipitation of TiC in the ferrite matrix is very important for producing a hot-rolled titanium steel plate having improved formability and toughness at low temperatures.
Encouraged by these observations, the present inventor continued his studies and found that a hot-rolled titanium steel plate whose blank with as-shorn edges has far better formability than the conventional TiC precipitation-hardened steel can be produced by meeting the following requirements:
(i) the steel is hot-rolled with a high reduction in thickness in a low temperature range so that TiC incoherent with the ferrite matrix is uniformly precipitated throughout the structure, i.e. the TiC is precipitated not only in the ferrite grains, but also in the grain boundary by the time the hot rolling is completed, and the precipitation hardening by this type of TiC is used to increase the strength of the steel; and PA0 (ii) after rolling, the steel is quenched in order to minimize the amount of coherently precipitated TiC and to increase the amount of Ti in solid solution, which accelerates the formation of a bainitic structure, resulting in a remarkable increase in the strength of the matrix.
Thus, according to the findings of the present inventor, a hot-rolled titanium steel plate in which the amount of titanium incoherently precipitated is restricted to being as large as possible, e.g. to not less than 0.02% by weight and the amount of the titanium coherently precipitated is restricted to being as small as possible, e.g. to not more than 0.015% by weight exhibits improved cold formability as well as toughness including improved resistance to cracking during bending a blank with as-shorn edges. Furthermore, in order to increase the amount of the incoherently precipitated titanium, it is necessary to apply hot rolling at a relatively low temperature with a high reduction in thickness, e.g. in a temperature range of 800.degree.-900.degree. C. with a reduction of 30% or more. On the other hand, in order to reduce the amount of the coherently precipitated titanium and to accelerate the formation of a bainitic structure the presence of which can offset the redution in strength caused by decrease in the amount of coherently precipitated Ti, it is necessary to carry out low temperature coiling after hot rolling, e.g. coiling at a temperature of 500.degree.-200.degree. C.